Avoid assuming that we know the spelling of getopt_long's error messages.

Avoid assuming that we know the spelling of getopt_long's error messages.

I've had enough of "fixing" this test case.  Whatever value it has
is limited to verifying that pgbench fails for an unrecognized switch,
and we don't need to assume anything about what getopt_long prints in
order to do that.

Discussion: https://postgr.es/m/9427.1547701450@sss.pgh.pa.us